All of the aluminum structure requires only assembly. You will need to design and add small items like doublers for antennas, instrument mounting. The instrument panel is a single slab of aluminum. Cutting holes and mounting avionics is for you to figure out. Further enhancements are yours to figure out as well.
Major flight systems are all designed and need only installation with the exception of avionics and the electrical to support it. You will need to choose the electrical architecture, wire routing and how to install into the airplane.
Lighting, both flight and interior, antennae placement, any engine features beyond the Lyc IO540 such as electronic ignition is again yours to figure out.
The interior is very basic with front seat foam and seating structures. Upholstery, including rear seat foam, and enhancements and creature comforts will be on you.
In fact, when I sat in the Vans RV-10 demo at Oshkosh, I almost decided not to build one because it was too Spartan for my taste. Then seeing the beautiful examples from builders, I saw the amazing potential of personalization possible.
My build will end up approximately 50% Vans plans and 50% stuff I decided I want to do.